This invention relates in general to dual actuator systems for positioning one part relative to another and, more particularly, to a method and apparatus for positioning a read/write head relative to a hard disk using a voice coil motor and a microactuator.
A hard disk drive typically includes a rotating magnetic disk and a read/write head supported adjacent one side of the disk for approximately radial movement relative to the disk. Data on the disk is organized in the form of a plurality of concentric tracks, each track being subdivided into a plurality of arcuate sectors that are circumferentially distributed. Each track also includes servo information which can be read by the read/write head, which identifies the particular track, and which also indicates the extent to which the read/write head is or is not accurately radially aligned with that track.
The read/write head is typically supported on a movable support arm, and an actuator such as a voice coil motor is provided in order to effect movement of the support arm. When the support arm is moved, the read/write head thereon is moved in a direction approximately radially of the disk. A control system is responsive to the servo information read from the disk by the read/write head for controlling the voice coil motor so as to position the support arm in a manner that radially aligns the read/write head with a selected track on the disk.
The capacity of hard disk drives is progressively increasing, due in part to a progressive increase in the number of concentric tracks provided on a given hard disk. Of course, the radial widths of the tracks decrease as the number of tracks is increased. As a result, there has been an increase in the precision and resolution needed for controlling the radial position of the read/write head in order to keep it aligned with a particular track. A further consideration is that, as central processing units become progressively faster, there is an associated increase in the need for hard disk drives with faster seek and access times.
One proposed approach for achieving greater precision and resolution while reducing seek and access times involves the use of a microactuator to movably support the read/write head on the support arm. Microactuators are miniature actuators or motors, which may be fabricated on silicon using semiconductor fabrication techniques, and which are sometimes referred to as microelectromechanical systems (MEMS). A microactuator is capable of effecting rapid and accurate movement of the read/write head relative to the support arm, in a direction approximately radially of the disk, but within a relatively small range of movement. The voice coil motor is thus used to move the support arm to effect coarse positioning of the read/write head, and the microactuator is used to effect fine positioning of the read/write head.
The servo information read from the disk by the read/write head identifies only the position of the read/write head relative to the disk. In a typical hard disk drive system without a microactuator, the read/write head is fixedly supported on the support arm, and thus the position of the support arm is directly related to the position of the read/write head. On the other hand, when a microactuator is provided between the support arm and the read/write head, the microactuator facilitates movement of the read/write head relative to the support arm. Thus, knowledge of the actual position of the read/write head based on the servo information read from the disk provides no information at all regarding the actual position of the support arm.
Accordingly, it has been considered necessary to supplement the position information from the read/write head with a sensor that determines the actual position of the support arm, either by directly sensing the position of the support arm, or by sensing the amount of relative movement effected by the microactuator between the support arm and read/write head. However, the need to provide such a sensor decreases the reliability of the system, while increasing its costs. In this regard, as storage capacity increases and the number of tracks increases, the actual position of the support arm must be determined with progressively increasing resolution and precision, which in turn involves increased cost and complexity for the sensor and associated circuitry that are provided to detect the actual position of the support arm. Consequently, while existing hard disk drives which use microactuators have been generally adequate for their intended purposes, they have not been satisfactory in all respects, due in part to the need to provide a sensor and supplementary circuitry.
From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for controlling a dual actuator system with just a single source of position information. According to the present invention, a method and apparatus are provided to address this need, and involve: providing a first actuator to move a second part relative to a member; providing a second actuator to effect movement of the member relative to a first part, the first and second actuators each effecting relative movement of the first and second parts; receiving an input signal that specifies a target position of the second part relative to the first part; generating a first actuator control signal as a function of the input signal and without sensing an actual position of the member, the first actuator control signal causing the first actuator to move the second part toward the target position with respect to the first part; and generating a second actuator control signal as a function of the first actuator control signal and without sensing an actual position of the member, the second actuator control signal causing the second actuator to move the member in a manner so that the second part moves toward the target position with respect to the first part.